BB, I am with Fade and Jim on this, be honest and truthful with your son. I am not saying throw your W under the bus, but I am saying to have a heart-to-heart with him and explain what happened. When I was a kid my parents were having trouble and I didn't know it until my dad pulled me aside and told me. He did not blame my mom for anything, he simply told me the facts. He said he didn't understand what was happening with her but that she was being very cold and distant. She had started coming home and going into the bedroom and closing the door and not interacting with any of us. He said he got a call from an apartment complex wanting to confirm the lease, he said he could only assume she was planning on moving out but he didn't know when or why. That had to be one of the hardest conversations he's ever had with me, maybe the hardest. He didn't get emotional, he didn't blame, he didn't accept guilt, he simply shared what he knew with me and treated me like I was mature enough to understand.
Do the same for your son, he will appreciate you more than ever.